The Romulan homeworlds orbit 128 Trianguli, otherwise known to the inhabitants as Eisn. The system consists of six planets. The two innermost planets are tiny cinders that orbit almost close enough to be devoured by the star. A wide asteroid belt separates these inner two planets from the next two, ch'Rihan and ch'Havran. There is another asteroid belt and then two gas giants orbit the outer reaches of the system.
Although ch'Rihan and ch'Havran orbit around a common axis, neither is tidally locked to the other. Despite willful misinformation and propaganda, there is no more a permanently dark side of either planet than there is a dark side of Earth's moon. A day on both planets is roughly the same, equal to nearly 25 Federation hours. A year on these planets is equal to not quite 346 Federation days.
ch'Rihan is lush and full of vegetation. Although hot and humid for half the year, it can be quite dry and cold during the winter. Many native beasts, such as hlai and sseikea, are native to the planet. Some of these, such as hlai, are eaten by the Romulans. Much of the vegetation is also edible, such as lehe'jhme which is distilled into Romulan wine.
ch'Havran is much drier. Due to a slight difference in atmospheric content, it is also significantly warmer than ch'Rihan. Although it also has large oceans, ch'Havran's climate comes close to that of Vulcan. In certain places, it even rivals Vulcan for inhospitability.
The oceans of both planets teem with a wide variety of plant and animal life. Although such an idea was a foreign concept to their Vulcan ancestors, Romulans have taken to the idea of seafood with gusto. A giant mollusk is farmed in the shallow waters of ch'Rihan while both planets enjoy eating both fresh and saltwater fish.
Both planets have a variety of oceans and seas. Together, the two worlds have a total of twelve continents. While ch'Rihan is lush and composed of approximately half land and half water, ch'Havran is closer to 60 percent land and 40 percent water. Several of their oceans are miles deep and, though they have the equipment to do so, the bottoms still have not been fully mapped by the Romulans.
ch'Havran has several tall mountain ranges that surround the largest ocean. These mountains block moisture that would otherwise be brought by the prevailing winds, creating large swaths of desert on the Eastern continent. The choicest pieces of livable land on ch'Havran lie much closer to the poles than they do on Earth.
The ice caps on both planets are relatively small in size but are still over a mile thick in places.
Having abandoned one harsh planet, the Romulans have no desire to ruin the near-perfect ecosystems of ch'Rihan and ch'Havran. They take environmental friendliness to an art. Factories are made aesthetically pleasing and are forbidden to release anything more dangerous than steam into the air or mildly warm water into the streams. Toxic waste may end up being shot into Eisn, rather than risk polluting the Hearthworlds.
Even the great mines of ch'Havran are invisible from space without a deep scan. Among other things, Mine tailings have been detoxified and sculpted into natural-looking mountains. If not for obvious signs of space travel and radio traffic, the Federation would have thought that the Romulans were a pre-industrial society upon discovery.